Mutual diffusivities at 25 degrees C were measured in the range of the polymer weight fraction of 0.15-0.5, using differential interference microscopy. Diffusivities of atactic polystyrene in toluene at 25 degrees C for 0.35-0.5 polymer weight fractions showed good agreement with the prediction of Vrentas and Duda. Polyamides (nylon-6, nylon-66, and a nylon-6, -66, and -610 amorphous terpolymer) in formic acid all exhibited very similar diffusivities. Although slightly higher in magnitude, the mutual diffusivities of cellulose acetate (39.8 wt % acetyl) in acetone had a compositional dependence that was parallel to the data for the polyamides. In the range of composition studied, there was an unanticipated very weak dependence of mutual diffusivity on the polymer weight fraction.